Method and apparatus for control of a shape memory alloy actuator for a fuel injector

ABSTRACT

A shape memory alloy actuator assembly for a fuel injector, wherein the response times of the shape memory alloy element is decreased to less than about 1 millisecond by forced, convective heat transfer from the SMA element or elements. The forced, convective heat transfer is provided by the circulation of fluid across the SMA element by a metering orifice plate, which directs a fluid flow across the SMA element so as to maximize the area of contact between the fuel and the SMA element, regardless of whether the fuel injection valve is opened or closed. Use of forced convective heat transfer in accordance with the present invention allows greater power input levels than previously possible without resulting in an over-temperature condition of the SMA alloy, as well as constant response times.

TECHNICAL FIELD

The present invention relates to a shape memory alloy actuator for afuel injector, wherein the response time of the actuator is improved byproviding forced, convective heat transfer to the shape memory alloyactuator.

BACKGROUND OF THE INVENTION

Certain metals commonly referred to as shape memory alloys exhibitcharacteristic material properties that make them desirable for use inactuators. Shape memory alloy actuation provides greater force pervolume than electromagnetic-type actuation, and is also less complex.These characteristics make shape memory alloy actuation highly desirablefor use in fuel injectors, particularly automotive fuel injectors.

Shape memory alloys (hereinafter "SMAs") undergo a temperature-relatedphase change which is characterized by the memory of any mechanicalconfiguration imposed on the material in its austenitic crystallinephase. In particular, SMAs have two different crystal structures thatare determined by temperature. In its low temperature state the materialexhibits a martensitic crystal structure which has a relatively lowmodulus of elasticity, and which can be easily deformed. However, whenthe alloy is heated above a temperature threshold, the transitiontemperature, its crystal structure changes to austenite and the alloyreturns to its original configuration.

This temperature-dependent memory characteristic is exploited inactuators for fuel injectors by providing a bias mechanism, for examplea spring, to deform the SMA element while it is in the low temperaturestate, then raising the SMA element's temperature, for example byresistance heating, in order to induce a return to the element'soriginal configuration. The return to the SMA element's originalconformation thereby creates motion in the spring, which in conjunctionwith the remainder of the actuator assembly results in opening orclosing of the fuel injector valve. Cooling of the SMA element returnsthe element to its low temperature, easily deformed phase. The biasspring force results in mechanical motion in the actuator which closesor opens the fuel injector valve. A major challenge in the use of SMAsin automotive fuel injectors has been to reduce the response time of thealloy so that the opening or closing cycle of the actuator is reduced toone millisecond or less. This fast response time is required in order toprovide the necessary minimum flow control necessary under light loadengine conditions.

It is known in the art that the response time is affected by the rate ofheat transfer (i.e., cooling) of the SMA element, and that the geometryof the alloy element has a direct affect on this heat transfer rate. SMAactuator geometries comprising small-diameter wires, ribbons, or thinfilms, for example, have been shown to maximize the heat transfer rateof the alloy, thereby achieving faster response times. Such geometrieshave been described in U.S. Pat. No. 4,806,815 to Homma; U.S. Pat. No.4,973,024 to Homma; U.S. Pat. No. 5,061,914 to Busch, et al.; U.S. Pat.No. 5,211,371 to Coffee; and U.S. Pat. No. 5,325,880 to Johnson et al.The width-to-thickness ratios disclosed in the prior art are in therange from 50:1 to 4:1, and resulted in best minimum response times ofabout 10 milliseconds. However, none of these geometries yield therequisite degree of heat transfer effective to provide response times atthe 1 millisecond level required for fuel injector applications.

It is further known in the art that the response time is affected by theenergy input (e.g., resistance heating) into the SMA element.Ordinarily, a high energy input into the SMA element is desirable, inorder to decrease the response time. This energy input has an inherentlimitation, however, due to the nature of the materials suitable forshape memory alloys. An "over-temperature" condition results in strainrecovery loss or destruction of the alloy. The response time of SMAactuators in the prior art have accordingly been limited in the amountof input power which may be applied to the SMA elements, and again, arelimited to response times of no less than 10 milliseconds. There thusremains a need in the art for economical methods and apparatus forcontrolling the operating conditions of shape memory alloy actuators forfuel injectors so as to provide response times of less than 10milliseconds, and preferably less than about 1 millisecond.

SUMMARY OF THE INVENTION

The above-discussed and other drawbacks and deficiencies of the priorart are overcome or alleviated by the method and apparatus of thepresent invention, wherein the response time of an SMA actuator assemblyfor a fuel injector is decreased to less than 10, and even to less thanabout 1 millisecond by forced, convective heat transfer from the SMAelement or elements. The forced, convective heat transfer is provided bythe circulation of fluid across the SMA element by a metering orificeplate and housing design, which direct a fluid flow across the SMAelement so as to maximize the area of contact between the fuel and theSMA element regardless of whether the fuel injection valve is opened orclosed. Use of forced, convective heat transfer in accordance with thepresent invention allows greater power input levels than previouslypossible without resulting in an over-temperature condition of the SMAalloy.

In another embodiment of the present invention, the response time of theactuator is adjusted or optimized by controlling at least one of theconvective heat transfer coefficient, the fluid flow path across theactuator, the fluid flow rate across the actuator, the thickness of thethermal boundary layer adjacent to the SMA element, the maximumtemperature reached by the SMA element, the ambient temperature of thefluid, the circulation rate of the fluid, and the temperature differencebetween the actuator and the ambient fluid.

In still another embodiment of the present invention, the minimum liftof the valve is adjusted so that any variation above this minimum has nosignificant effect on the flow rate of fluid through the valve, and theinput power into the SMA element is controlled in order to maintainconsistent maximum material temperature, thereby maintaining relativelyconsistent reverse transformation response times. Maintenance ofconsistent response times results in minimum flow rate shifts and thusenhanced fuel injector operation.

The above-discussed and other features and advantages of the presentinvention will be appreciated and understood by those skilled in the artfrom the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings wherein like elements are numbered alikein the several FIGURES:

FIG. 1 is an plan view of a an actuator assembly of the presentinvention showing the length (l) and width (w) dimensions of the SMAelement.

FIG. 2 is an isometric view of an actuator assembly of the presentinvention comprising a metering orifice plate as shown in FIG. 3, avalve and SMA elements.

FIG. 3 is an isometeric view of a metering orifice plate of the presentinvention, showing unmetered flow paths.

FIG. 4 is a cross-sectional view of the actuator assembly of the presentinvention with the valve and the metering orifice plate in the closedposition, and showing a perpendicular recirculation flow path.

FIG. 5 is a cross-sectional view of the actuator assembly of the presentinvention with the metering orifice plate in the opened position andshowing a perpendicular recirculation flow path.

FIG. 6 is a cross-sectional view of an alternative embodiment of theactuator assembly of the present invention with the valve in the closedposition and showing a parallel recirculation flow path.

FIG. 7 is a cross-sectional view of an alternative embodiment of theactuator assembly of the present invention with the valve in the openposition and showing a parallel recirculation flow path.

FIG. 8 is an oscilloscope trace of (A) the actuator position vs. timeand (B) the supply power logic trace, wherein the position traces arewith and without the circulation flow induced forced convection of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In accordance with the present invention, the response time of an SMAactuator assembly for a fuel injector is decreased by providing forced,convective heat transfer from the SMA element or elements. Optimizationof the response time is controlled by controlling at least one of thefuel flow paths across the actuator, the fuel flow rate across theactuator, the thickness of the thermal boundary fuel layer adjacent tothe SMA element, the maximum temperature reached by the SMA element, theambient temperature of the fuel, the circulation rate of the fuel, andthe temperature difference between the actuator and the ambient fluid.The use of forced, convective heat transfer also allows greater powerinputs to the SMA elements than previously possible.

As used herein, "forced" convective heat transfer refers to convectiveheat transfer caused by fluid having a flow directed so as to contactthe shape memory alloy, then directed away from the shape memory alloy,thereby increasing the convective cooling of the shape memory alloy.This forced, convective heat transfer is in lieu of, or additional to,any convective heat transfer that occurs by the mere presence of fluidsurrounding the shape memory alloy, or by fluid that is being meteredthrough the valve of the fuel injector. Use of forced, convective heattransfer uniquely reduces the response times of the SMA element orelements to less than 10 milliseconds, preferably less than about 5milliseconds, and even more preferably to less than about 1 millisecond,which is the level required for automotive fuel injector applications.

Forced, convective heat transfer using fuel as a transfer medium isprovided by the apparatus of the present invention shown in FIGS. 1-7,wherein the flow of metered fluid is in the range from about 1.0 toabout 12.0 g/sec. Although the FIGURES and the following discussion showthe valve in the closed position at ambient fuel temperature, and in theopen position upon application of resistance heat to the SMA elements,it is to be understood that the reverse is equally within the scope ofthe present invention, that is, depending on the original, undeformedconfiguration of the SMA elements, the valve may be in open position atambient fuel temperature, and closed upon the application of heat to theSMA elements.

Accordingly, an actuator assembly 10 for an automotive fuel injectorcomprises a valve 20, and SMA element 22 in contact with valve 20. AllSMA materials presently known are suitable for the practice of thepresent invention. As shown in FIG. 1, SMA element 22 generally has awidth w, a length l, and a thickness which in this view is into thepaper. SMA element 22 preferably has a minimum width-to-thickness ratiogreater than about 4:1, and preferably greater than about 500:1.Metering orifice plate 30 comprises a flow metering orifice 32 andoutlet flow paths 35, 36 disposed on either side of metering orifice.

In FIG. 4 valve 20 is in the closed position at ambient fueltemperature. Application of an electrical current to SMA element 22results in resistance heating, which raises the temperature of SMAelement 22 to above the transition temperature. Conversion of thecrystal structure to the austenite phase results in the return ofelement 22 to its undeformed position, which results in valve 20 beingmoved to the open position, and flow of metered fuel through flowmetering orifice 32 as depicted in FIG. 5. As shown in FIGS. 4 and 5,the forced, convective current of fuel through inlet flow path 34 acrossSMA element 22 and through outlet flow paths 35, 36 increases the heattransfer rate of SMA element 22. The forced, convective current of fuelexits the actuator assembly 10 through return path 38 which is in fluidcommunication with outlet flow paths 35, 36. Preferably, flow paths 34,35, 36 direct the fuel flow path so as to maximize the area of contactbetween the fuel and SMA element 22. In an important feature of theinvention, circulation of fuel across SMA element 22 is effected viaflow paths 34, 35, 36 independently of whether valve 20 is in an open orclosed position across flow metering orifice 32. Injector closingresponse time may be reduced to 0.6 milliseconds with use of forced,convective heat transfer as described above. This short response time isparticularly advantageous for fuel injector applications.

While the above-described configuration for fuel flow paths 34, 35, 36is preferred, as it is effective to provide maximum heat transfer usingminimum fluid volume circulating across the SMA element, it is to beunderstood that other arrangements of inlet and outlet orifices and fuelflow paths are effective and are within the scope of the presentinvention. Thus, where FIGS. 4 and 5 provide for an inlet flow pathoriented perpendicularly to the shape memory alloy element, it is alsowithin the scope of the invention to provide an inlet flow path orientedparallel to the shape memory alloy element as is shown in FIG.5.

As with other SMA-based actuators, the opening response time for theinjector shown in FIGS. 1-5 is a function of the input power to the SMAelement, and the heat transfer rate away from the element. The closingresponse time is a function of both the amount of energy to be removedand the heat transfer rate from the actuator. Use of forced convectiveheat transfer in accordance with the present invention provides severalmechanisms whereby the opening and closing response times may beadjusted and optimized, that is, by control of at least one of theconvective heat transfer coefficient, the fuel flow path(s) across theactuator, the fuel flow rate across the actuator, the thickness of thethermal boundary layer adjacent to the SMA element, the maximumtemperature reached by the SMA element, the ambient temperature of thefuel, the circulation rate of the fuel, and the temperature differencebetween the actuator and the ambient fluid. Final optimization of theresponse time for an SMA actuator may be both modeled and empiricallydetermined, and will ultimately depend on competing considerations, someof which are outlined below.

For example, different actuator geometries will require different flowpaths and flow rates to obtain the optimum heat transfer rate. Ideally,the convective fluid is directed across the SMA elements in thedirection which gives the greatest fluid contact surface area.

In addition, the fluid velocity must be sufficient to move the fluidacross the alloy in a time faster than the targeted response time. Thisallows the removal of energy away from the alloy and prevents vaporgeneration around the alloy, which would lower the overall heat transferrate. The rate of heat transfer is a particularly importantconsideration, in that if it is too great, the actuator will requirelarge input power levels to reach the crystal structure phase changetemperature. If the heat transfer rate is too low, the input power mustbe decreased to avoid a material over-temperature condition, and theopening and closing response times are increased.

Use of forced convective heat transfer from the SMA element not onlyallows a decrease in the response time via an increase in the convectiveheat transfer from the SMA element, but also allows optimization of theinput power to the SMA actuator. As discussed above, the openingresponse time for a fuel injector comprising an SMA actuator isdependent not only on the heat transfer rate from the actuator, but alsoon the input power to the SMA elements, for example in the form ofresistance heating. The amount of heating has heretofore been limited toan amount which will not result in an over-temperature condition. Themethod in accordance with the present invention allows greater heattransfer away from the SMA element, and therefore greater power inputwithout the risk of SMA over-temperature.

FIG. 8 is an oscilloscope trace of (A) the actuator position vs. timeand (B) the supply power logic trace. The position traces are with andwithout forced convective heat transfer, and thus have different heattransfer rates. The traces indicate that a change in the slope of theopening and closing events occurs as a result of the circulation flowrate addition. The time required to deactuate the actuator is reduceddue to the increase in heat transfer rate from the fuel flow path.Furthermore, the time required to actuate the actuator is reduced due tothe fact that the input power is increased as a result of the increasedheat transfer rate, while the risk of alloy over temperature is reduced.

However, the level of input power also requires consideration of thefactors which control the response time for the closing (cooling) cycle.The heat transfer rate for the closing cycle is determined by theconvective heat transfer coefficient and the temperature differencebetween the SMA element and the ambient fluid. Control of the SMA peaktemperature to that just above the full austenite finish transformationtemperature results in the minimum energy to be removed for cooling, andthus shorter closing response times. Optimal closing time is thusobtained by adjusting the SMA element temperature to just above thatrequired to cause the return to the original, undeformed configuration.This implies that optimal closing time is achieved by restricting theinput power to a minimum. But because the forced convective heattransfer of the present invention increases the convective heat transfercoefficient, the heat transfer rate allows both an increase in inputpower and a reduction in the opening response time, as well as decreasedclosing response times.

In a preferred embodiment of the present invention, the peak temperatureof the SMA element is further controlled in order to maintain constantresponse times, regardless of ambient fuel temperature variations. Inthis embodiment, the minimum valve travel (minimum distance between thevalve and the valve metering orifice when the valve is in the openposition) is such that any variation above this minimum has nosignificant effect on the flow rate of the fluid through the meteringorifice, i.e., the pressure drop is only across the metering orifice.This embodiment is particularly advantageous in that maintainingconstant response times with sufficient valve travel results in minimumflow rate shifts regardless of any changes in ambient fluid temperatureor convective cooling rates.

As shown in FIGS. 6 and 7 inlet flow enters via flow path 34 above SMAfilm element 22, directing fuel to maximize contact area across elements22. The forced, convective current of fuel exits the actuator assembly10 through return path 38. The flow velocity is such that a sufficientvolume of fluid is directed to and away from the SMA elements to affectheat transfer in a time period preferably less than the desired responsetime. Thus for an SMA element of 3 mm length, having a width of 0.50 mmand a thickness of 0.010 mm, a flow path directing fluid across thelength of the SMA element (FIGS. 6 and 7) at a velocity of 3meter/second results in the fluid traversing the length of the SMAelement in 1 millisecond. A flow path directing fluid across thethickness of the SMA element results in the fluid traversing thethickness of the SMA element in 3 microseconds. In these embodiments theflow paths direct the fluid along and across the SMA elements length andthickness. Proper selection of the base circulation flow rate throughthe actuator results in response times of less than about onemillisecond. Proper selection of flow rate and flow path results in athermal boundary layer of approximately 1.5 times the thickness of theSMA element.

In another embodiment of the present invention, the fluid flow paththrough assembly 10 and across SMA element 22 is such that the meteredflow provides convective heat transfer effective to result in an optimalresponse times of less than about 1 millisecond. This embodiment is theoptimum configuration with respect to input power, since when the valveis in the closed position, there is no enhanced convective cooling, andthus the heat transfer away from SMA element 22 and the input power isat a minimum. When the valve is in the open position, the metered flowfollows a path which results in enhanced convective heat transfer, andis of sufficient velocity such that response times equal or less thanapproximately 1 millisecond are realized.

While preferred embodiments have been shown and described, variousmodifications and substitutions may be made thereto without departingfrom the spirit and scope of the invention. Accordingly, it is to beunderstood that the present invention has been described by way ofillustration and not limitation.

We claim:
 1. A shape memory alloy fuel injector having forced convectiveheat transfers, comprising:an actuator assembly including a valve and atleast one shape memory alloy element in contact with the valve, theshape memory alloy element further having a characteristic phase changetransition temperature wherein the shape memory alloy element and thevalve have a first position at a temperature below the transitiontemperature and a second position at a temperature above the transitiontemperature; and a metering orifice plate including a flow meteringorifice which is blocked by the valve at either the first position orsecond position of the shape memory alloy element and the valve, therebyproviding a metered flow of fluid through the fuel injector; at leastone inlet flow path and at least one outlet flow path, wherein the atleast one inlet flow path and the at least one outlet flow path direct aflow of fluid independent from the metered flow of fluid across theshape memory alloy element, thereby providing a thermal boundary layerbetween the shape memory alloy element and the bulk flow of fluid thatresults in an enhanced convective heat transfer rate from the shapememory alloy element.
 2. The shape memory alloy fuel injector of claim1, wherein:the shape memory alloy element has a width-to-thickness ratiogreater than about 4:1.
 3. The shape memory alloy fuel injector of claim2, wherein:the shape memory alloy element has a width-to-thickness ratiogreater than about 500:1.
 4. The shape memory alloy fuel injector ofclaim 3, wherein:the configuration of the actuator assembly results inmovement of the valve from one position to the other position in lessthan about 1 millisecond.
 5. The shape memory alloy fuel injector ofclaim 1, wherein:the shape memory alloy element is orientedperpendicular or parallel to the inlet flow path.
 6. The shape memoryalloy fuel injector of claim 1, wherein:the flow path is effective toprovide maximum heat transfer using minimum fluid volume circulatingacross the shape memory alloy element.
 7. The shape memory alloy fuelinjector of claim 1, wherein:the thermal boundary layer is about 1.5times the thickness of the shape memory alloy element.
 8. The shapememory alloy fuel injector of claim 1, wherein:the minimum valve travelis such that a greater distance between the valve and the valve orificehas no significant effect on fluid flow rate through the meteringorifice.
 9. A method for optimizing the response time of a shape memoryalloy fuel injector, comprising providing a fuel injector including anactuator assembly including a valve and at least one shape memory alloyelement in contact with the valve;the shape memory alloy element furtherhaving a characteristic phase change transition temperature, wherein theshape memory alloy element and the valve have a first position at atemperature below the transition temperature and a second position at atemperature above the transition temperature, the movement of the valvebetween the first and second positions providing a metered flow of fluidthrough the fuel injector; and further wherein the time required for theshape memory alloy element and valve to move from one position to theother position is the response time; and forcing fluid flow around theshape memory alloy element independently of metered fluid flow, therebyincreasing the convective heat transfer coefficient of the fuelinjector.
 10. The method of claim 9, wherein: the independent fluid flowaround the shape memory alloy element is directed in a flow path by ametering orifice plate including a flow metering orifice which isblocked by the valve at either the first position or the second positionof the shape memory alloy element and the valve, at least one inlet flowpath and at least one outlet flow path.
 11. The method of claim 9,wherein: the shape memory alloy element has a width-to-thickness ratiogreater than about 4:1.
 12. The method of claim 11, wherein: the shapememory alloy element has a width-to-thickness ratio equal to or greaterthan about 500:1.
 13. The method of claim 9, wherein: the flow path iseffective to provide maximum heat transfer using minimum fluid volumecirculating across the shape memory alloy element.
 14. The method ofclaim 9, wherein: the forced fluid flow creates a thermal boundary layerbetween the shape memory alloy element and the bulk of the fluid flow.15. The method of claim 14, wherein: the thermal boundary layer is about1.5 times the thickness of the shape memory alloy element.
 16. Themethod of claim 9, wherein: the minimum valve travel is such that agreater distance between the valve and the valve orifice has nosignificant effect on fluid flow rate through the metering orifice. 17.The method of claim 9, wherein: the response time of the shape memoryalloy element is adjusted by controlling at least one of the convectiveheat transfer coefficient, the fluid flow path across the actuatorassembly, the fluid flow velocity across the actuator, the thickness ofthe thermal boundary layer adjacent to the shape memory alloy element,the maximum temperature reached by the shape memory alloy element, theambient temperature of the fluid, the circulation rate of the fluid, thedemetered flow rate, and the temperature difference between the actuatorand the ambient fluid.
 18. The method of claim 9, wherein: the responsetime of the shape memory alloy element is adjusted by controlling thefluid flow path across the actuator assembly, the fluid flow velocityacross the actuator assembly, the thickness of the thermal boundarylayer adjacent to the shape memory alloy element, and the maximumtemperature reached by the shape memory alloy element.
 19. The method ofclaim 9, wherein: the response time of the shape memory alloy element isless than about 1 millisecond.
 20. The method of claim 9, wherein: inputpower into the shape memory element causes the shape memory alloyelement to attain the temperature above the transition temperature, andfurther wherein the input power level is controlled to optimize responsetime and prevent an over-temperature condition.
 21. The method of claim14, wherein: input power into the shape memory element causes the shapememory alloy element to attain the temperature above the transitiontemperature, and further wherein the input power level is controlled soas to maintain constant response times.